Plaster an old brick wall

I need to re-plaster part of a brick wall in the living room of a Federation house, where the orginal plaster/render behind a bathroom shower has lifted from the brick wall. The wall is now dry (shower plumbing fixed), but I suspect that it is wrong to use a normal modern render mix under the final plaster layer, as it will almost certainly be too hard when dry for such things as picture hook pins, and to blend properly with the layers that are still fixed to the wall. Does anyone know what might be a reasonable render mix to use? Although I have not yet tried to remove it, I also suspect that the original is not far from a lime based render.

Yes,it will almost certainly be a lime based mix. Traditionally this type of plaster was put on in three coats, the first with sharp sand , hair and lime in the ratio 3:1 , the second coat fine sand and lime 2:1 and the last coat straight lime putty.
Have a look at the link below in the documents and publications area for some more info.
www.heritage.nsw.gov.au
Regards,
Tote